1. Field of the Invention
The present invention relates to a method of driving a recording head adapted to effect recording on a recording medium by using heat. It also relates to a recording apparatus utilizing this method.
The above recording apparatus may, for example, be an electronic typewriter, copying machine, facsimile machine, or printer or the like. The recording system having a recording head adapted to effect recording by using heat may, for example, be (1) the so-called ink-jet system in which a change of state is caused by heat in the recording liquid and, in accordance with this change, the recording liquid is expelled through discharge nozzles so as to form a spray of liquid drops; (2) the so-called thermosensitivity system according to which thermosensitive paper is caused to change to color by heating it; or (3) the so-called thermal-transfer system according to which the ink retained on an ink ribbon is selectively transferred to the recording medium by means of heat.
2. Description of the Prior Art
Recording apparatus such as printers and facsimile terminal equipment are designed to form dot patterns on recording sheets (recording media such as recording paper, thin plastic plates or the like) by selectively driving in accordance with recording information (e.g., image data signals) a plurality of dot-forming elements provided on the recording head. That recording apparatus may be of the serial type in which recording is performed while moving the recording head along the sheet-width direction, the line printing type in which recording is effected collectively for a predetermined length in the line direction, or the page printing type in which recording for an entire page is effected simultaneously.
The recording system used may be the thermal system, the ink-jet system, the wire-dot system or any other system utilizing dot-forming elements. Of these, the thermal system can be classified into the thermal-transfer system in which ink is transferred to a sheet of ordinary paper by means of an ink sheet, and the thermosensitivity system in which thermosensitive paper changes color upon being heated.
Conventionally, a medium-tone recording system for indicating differences in density has been employed when performing color recording or image recording using a plurality of colors such as cyan, magenta, yellow and black. These systems use an area-gradation method, such as the Dither method, which is based on the principle of binary recording for expressing gradations. In that method, a plurality of dots are regarded as one unit and the medium tone is determined by the proportion of ON and OFF dots (binary recording) in that unit, thus giving the appearance of an image of medium-tone dots, when in fact no medium-tone does exist.
The problem with the above-described area-gradation method is that the requisite dot number with respect to one dot must be increased before many gradations can be expressed, which results in the image resolution being deteriorated. To obtain an image of a resolution of about 6 pixels/mm with 64 gradations, about 48 dots/mm must be generated by the recording head. To realize this with a thermal printer, a thermal head of 48 dots/mm is required. However, it is difficult to manufacture thermal heads of such high density. Even if such a thermal head were realized, it would be rather impractical since it would have to have an enormous number of elements, which means a large-scale driving circuit would be necessary to drive this thermal head. Thus, there is a limit to the quality of the gradation-recording-images obtained by binary recording. Hence, there is a demand for a practical multivalue-gradation-recording system in which a dot of one size is expressed in many gradations in some way or other.
In view of this, the inventors of the present invention have proposed in Japanese Patent Laid-Open No. 63-54261 (Japanese Patent Application No. 61-198188, filed on Aug. 26, 1986 and laid open to public inspection on Mar. 8, 1988) a thermal head which is capable of performing multivalue recording. This thermal head is designed such that the width of the electrode at its connection point with the heating element is equal to or smaller than the effective width of the heating element allowing recording. By virtue of this structure, the section of the heating element in the vicinity of the connection point between it and the electrode generates heat more intensely, thereby establishing selective heat distribution and making it possible to realize satisfactory multivalue gradation recording.
FIGS. 1 to 6 show the construction of a conventional thermal-head-driving apparatus for realizing gradation recording.
The apparatus shown includes a color-image input terminal 1 for separate color images of red, green and blue, a color conversion circuit 2 for converting input RGB (red, green and blue) signals into YMC (yellow, magenta and cyan) signals, a data line memory 3 for storing image data for each printed line, and a medium-tone control section 4. When printing at Gradation Level n (hereinafter referred to simply as "Gradation n"), pulses corresponding to Gradations 1 to n are sequentially supplied by this medium-tone control section 4. FIG. 2 shows an example of a pulse row output when the gradation is 10. The conventional apparatus further includes a control section 7 adapted to successively supply pulses corresponding to all gradations from the minimum to the maximum, in the order and with the respective pulse widths shown in FIG. 3, and a thermal head 8 consisting of heating resistor elements R.sub.1 to R.sub.n and adapted to dissolve or sublimate the ink or dye of an ink sheet 10 and to transfer it to a recipient sheet 11. The conventional apparatus further includes a temperature measuring element 9 for measuring the temperature of the thermal head 8, and a platen roller 12.
FIG. 4 is a circuit and block diagram of the medium-tone control section 4 shown in FIG. 1.
In FIG. 4, reference numeral 17 indicates a shift register adapted to convert serial-data signals for one line of data (1-line data) into parallel signals. Reference numeral 16 indicates a latch for latching the parallel signals obtained from shift register 17. A.sub.1 to A.sub.n represent AND gates adapted to output data signals only for the time corresponding to the pulse width of strobe signals. Q.sub.1 to Q.sub.n represent transistors adapted to drive the heating resistor elements R.sub.1 to R.sub.n in accordance with the pulse rows from the AND gates A.sub.1 to A.sub.n. The reference numeral 18 indicates a common electrode for applying voltage; 19 indicates an input terminal for strobe signals; and reference numeral 20 indicates an input terminal for data signals.
FIG. 5 is a timing chart showing an example of timing for the above-described medium-tone control section 4. This chart shows an example of the drive timing for the thermal-head 8.
In this conventional thermal-head driving apparatus, which has the above-described construction, pulses as shown in FIG. 6(a) are supplied from the medium-tone control section 4 to the thermal head 8 when lower density printing immediately follows higher density printing, for example, when performing printing at Gradation 2 immediately after printing at Gradation 10. However, since the temperature-time characteristic when the thermal head cools down to the initial temperature T.sub.0 .degree. C. is to be represented with an exponential function, the temperature T.sub.1 .degree. C. of the thermal head immediately before the printing at Gradation 2 is higher than the initial temperature T.sub.0 .degree. C. and there is the danger that printing at Gradation 2 will be at a higher density than is the proper density of that level.